That was before we got the update with the ciphers. I imagine the drop rate is the same. Also not sure if everyone knows you must have a psijic recipe in your inventory to get the cypher.
From the patch notes
Mythical Aetherial Ambrosia
We've introduced Mythic Aetherial Ambrosia to Tamriel! Mythic Aetherial Ambrosia is an incredibly rare, incredibly potent version of Psijic Ambrosia. Drinking it grants you a +150% bonus to the Experience you gain while it is in effect, and just like Psijic Ambrosia, its consumption and crafting is affected by your Connoisseur and Brewer passives. While Mythic Aetherial Ambrosia will stack with other foods or drinks, it will not stack with Psijic Ambrosia.
To craft Mythic Aetherial Ambrosia, you must first find an Aetheric Cipher.
This extremely rare item can potentially be found on any monster within certain areas, listed below.
You also need a Recipe Scroll for Psijic Ambrosia.
Use the Aetheric Cipher while the Psijic Ambrosia Recipe scroll is in your inventory, and the recipe scroll will transform into a Mythic Aetherial Ambrosia Recipe scroll.
This recipe can only be understood by those with the highest rank of the Recipe Quality Provisioner passive.
The recipe for Mythic Aetherial Ambrosia calls for one unit of Aetherial Dust and one Psijic Ambrosia drink.
Aetherial Dust is very rarely found on the resource nodes in certain zones - it can appear on ore, raw cloth, fallen wood, alchemical herbs and mushrooms, and water sources. It is not found on monsters.
So it appears you need a scroll of Psijic Ambrosia in your bag to make Mythical Aetherial Ambrosia
You must also learn the Psijic Ambrosia recipe to produce its reagent requirement.
So, 2xPsijic Ambrosia essentially if you dont already know it ???
Also not sure if everyone knows you must have a psijic recipe in your inventory to get the cypher.